--> Abstract: A Physical Explanation for the Depth to Top of Overpressure in the Gulf Coast Basin, by U. T. Mello and G. D. Karner; #90987 (1993).

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MELLO, ULISSES T., Lamont-Doherty Geological Observatory of Columbia University, Palisades, NY, Petroleo Brasileiro-Petrobras; and GARRY D. KARNER, Lamont-Doherty Geological Observatory of Columbia University, Palisades, NY

ABSTRACT: A Physical Explanation for the Depth to Top of Overpressure in the Gulf Coast Basin

In the Gulf Coast basin the depth to the Top of Overpressure ranges from 1.7 to 3.5 km in most of the shale dominated sequences. At present, the mechanism of overpressure generation is relatively well understood. However, there is no clear explanation as to why the depth to the Top of Overpressure depth should be relatively constant. We propose that the hydraulic diffusivity of a porous media is fundamental in controlling this depth. The hydraulic diffusivity, analogous to thermal diffusivity, essentially describes the rate of pressure change induced by the flux of fluid across aunit area of sediment and is defined as the ratio between the hydraulic conductivity and the specific storage. The variation of hydraulic diffusivity with depth has a minimum value that ranges from 2 to 4 km. t is this minimum that controls the shallowest position of the Top of Overpressure. Because sediment porosity and permeability tend to be high for shallow depths within a basin (i.e. <2 km), the hydraulic diffusivity will also be high. The reduction of permeability for these depths can feasibly induce changes in the hydraulic diffusivity of up to 3 orders of magnitude.

Below 4 km, the rate of specific storage decrease will be faster than the rate of hydraulic conductivity decrease and produces a relative increase of hydraulic diffusivity with depth. The decrease in specific storage with depth is due to the reduction of sediment bulk compressibility associated with the decrease of porous space and the increase of mechanical strength of the sediment as a function of depth. Within this 2-4 km minimum, the exact position of the Top of Overpressure will be controlled by secondary factors such as changes of lithology associated with facies variations and theactual distribution of intercalated sands. The Top of Overpressure cannot be sustained indefinitely with time and will tend to deepen when overpressure dissipated faster than it is generated.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.